Aromatic polyimides have high thermal resistance and good mechanical and electrical characteristics. On the other hand, however, they have problems in that since they are generally unmeltable and, moreover, insoluble in organic solvents of common use, hence they can be molded only with difficulty.
A method so far employed generally to solve such problems comprises synthesizing a soluble intermediate, such as a polyamic acid, by reacting an aromatic tetracarboxylic dianhydride with an aromatic diamine, which are starting materials for the manufacture of aromatic polyimides, in a polar organic solvent, giving a particular shape to the intermediate in that intermediate stage and thereafter dehydrating the same for ring closure at high temperatures to give a polyimide. However, this method is disadvantageous in that the intermediate has poor stability, that, therefore, standing at room temperature readily results in viscosity change and/or turbidity, and that since the dehydration reaction is conducted after molding, defects such as voids and pinholes tend to form and make it difficult to obtain surface-smooth and homogeneous moldings.
Accordingly, it is to be desired that a polyimide soluble in an organic solvent, readily moldable into films and so forth and free of the above-mentioned drawbacks should be developed. Thus, a variety of proposals have so far been made. For instance, a method has been proposed which comprises using, as the aromatic tetracarboxylic acid component, a diester form, which is less reactive with the aromatic diamine component than the dianhydride, to give a polyimide which has a low degree of polymerization and is soluble in solvents (Unexamined Japanese Patent Publication SHO 61-83229). However, this polyimide has a drawback that since its molecular weight is low, those heat resistance and mechanical characteristics which are intrinsic to polyimides are decreased in it.
A polyimide is also known which is soluble in solvents as a result of the use, as the aromatic diamine component, of a polynuclear m,m'-diamino compound with the terminal amino groups being in meta positions for the purpose of disturbing the symmetry of the polymer structure and the regularity in repeating units (Examined Japanese Patent Publication SHO 52-30319). However, this polyimide is disadvantageous in that due to the lack of uniformity in the polymer structure it has decreased heat resistance characteristics, namely lower thermal decomposition temperature and lower glass transition temperature. A further disadvantage is that the m,m'-diamino compound to be used is in itself very difficult to synthesize as compared with those p,p'-diamino compounds which are in general use as the aromatic diamine component and, furthermore, is inferior in reactivity to the p,p'-diamino compounds.
Furthermore, a method of producing soluble polyimides by using a p,p'-diamino compound has been proposed which comprises using combinedly two or more aromatic tetracarboxylic acid components differing in skeletal structure (Unexamined Japanese Patent Publication SHO 61-28526, Unexamined Japanese Patent Publication SHO 61-51033). However, it is a drawback of this method that the polymer structure becomes ununiform, hence it is difficult to obtain the same quality constantly. The quality may vary and the polyimide-proper toughness and electric characteristics can hardly be enjoyed to the full.
As mentioned above, all the known methods of solubilizing polyimides in organic solvents have the drawback that the excellent characteristics of aromatic polyimides proper are damaged as a result of giving solvent solubility to the polyimides.